The present invention relates to a method of measuring the surface of a workpiece using a motorised scanning head mounted on a coordinate positioning apparatus such as a coordinate measuring machine (CMM), machine tool, manual coordinate measuring arm and inspection robot.
It is known from International Patent Application No. WO90/07097 to mount a motorised scanning head on a coordinate positioning machine. The motorised scanning head enables a stylus mounted on the motorised scanning head to be rotated about two nominally orthogonal axes. The stylus may be positioned angularly about these two axes whilst the motorised scanning head can be positioned by the coordinate positioning machine in any position within the working volume of the machine.
Such a motorised scanning head provides a coordinate positioning machine with greater scanning flexibility because the motorised scanning head can position the stylus in many different orientations.
This application discloses measurement sequences in which simple movement of the coordinate positioning apparatus is combined with movement of the motorised scanning head to measure regularly shaped parts. For example a bore is measured by moving the quill of the CMM along a centre line while the motorised scanning head moves the stylus tip in a circular profile thus resulting in a helical motion.
This also discloses the measurement of a cone by driving the X and Y motors of the CMM to move the quill in a circular path while operating the M1 motor in bias mode and the M2 motor in the positioning mode. The biasing mode enables the tip to maintain contact with the surface while the scanning head is driven around the workpiece. This is done by applying to their rotors a substantially constant torque for a given input current.
This method of scanning features such as bores has the disadvantage that it is not suitable for large bores (i.e. if the radius of the bore is greater than the stylus length) or very small bores. Furthermore, this scanning method is not always possible due to access restrictions.